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Topic: How accurate should the "optical" path be?
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 03-22-2000 02:12 PM
You know how the center of a lens should line up with the (image) center of the film, and the reflector and lamp. How accurate should all of that be? Of course, the closer the better, but can anyone tell me exactly what the limit is, in the form of, " +/- .00X ."Also, how would you go about lining everything up? I'm thinking of an, "order of precedent." For example, the intermittent sprocket can't (usually) be laterally adjusted. So, maybe the first step would be to adjust the trap lateral guides to be directly in line with where the intermittent sprocket puts the film. Then, you might use that as a reference to align the lens; use the lens as a reference for something else, etc. This might be different for one projector to another. I can see how the lens could be lined up to the reflector (using test film) but how would you insure that everything else was lined up (lamp, film, seperate swing-away anamorphic adapter.)
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Pat Moore
Master Film Handler
Posts: 363
Registered: Mar 2000
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posted 03-24-2000 05:06 AM
Hi Guys;
For what we call the "optical A-Chain", the lens is the key for the whole system. Projector manufacturers take great care to be sure the lens holder(s) are properly aligned to the film trap and gate. Before you change those relative positions be sure you have a really good reason to do so. The alignment of the reflector in that A-Chain ia also a key, perhaps most important. It's easiest to use the lens as a starting point, using a laser alignment tool. OK, not everyone has one of those, but that's the best way.
The relfector keys are Working Distance (its relation to the film plane) and its axial alignment to the lens and film centerlines. The reflector's optical axis must be the same as the lens and film. If it's not, you'll have trouble with light output, distribution, and image quality on screen.
Lots more to talk about, but it's true that the image projectio system has an "A-Chain" just like a sound system. If it's not right, what you get in the B-Chain (screen and auditorium) will never be optimized.Pat
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 03-26-2000 05:06 PM
Okay, but still; does anyone know for sure how close the optical center should be?What do you do with a Cinemecannica V8, where neither the intermittent sprocket or the trap can be changed? I'm guessing a V8 is supossed to be machined correctly at the factory. One reason I'm asking is because I find that many of those "lens bushings" (to adapt a regular lens to a 4" holder) are way off. One from Century was out-of round by 0.015" which seems alot. On the Simplex bushings, they have two plastic screw things to hold the lens in, but also seem to hold the lens off center. Also, the lens holders in some Simplexes we have (PR1050) seem off. So, anyway, I was just wondering how close it should be. Maybe I'm getting too picky-picky, and 0.015" or so is still OK.
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 03-27-2000 09:52 AM
OK, if +/-0.015" is acceptable I'll go with that. I've been trying to get it to +/-0.005", and not very sucessfuly, either.I'll ask two other related questions: It is strongly implied (by projector mfg.s) that a 35/70mm projector can be changed from one format to another on-the-fly by just changing parts and settings, like the lens "mag-optical" setting. Yet, I never saw where the image was still centered on the screen after going from/to 35/70mm. The pedstal always had to be pushed left/right a little. Has anyone been able to from/to 35 or 70mm without moving the pedstal? I worked with a tech guy (at UA) who would remove and throw away the studio guides on every projector. Is that good or bad? Pat wrote:
"Projector manufacturers take great care to be sure the lens holder(s) are properly aligned..." Not the Centurys I worked with. Admittedly, it was 15 years ago, but then they were totally off. Actually, it was the gate/trap; Thread film and close the gate with a brand-new-out-of-the-box Century, and it would pinch the film so hard it would strip the sprocket, (good thing it was not polyester!) You could see the gate was on an angle- they just screwed the four screws in and shipped it. It's from working with those that I've become annoyingly suspicious of every optical path.
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 03-27-2000 03:40 PM
Sadly, the first Centurys I ever used were the same ones that made me dislike Centurys in general- which is probably unfair today. I tend to gauge all Centurys on these two machines. We had them rebulit in 1982, and had endless problems with them. After talking with several people, I got to talk to a guy named "Nick" in the drafting dept. who was very helpful. But then he quit. Then I talked to another guy who was good, but then he quit, too. They were JJ-2's with the eccentric bushing for the lens.One time I completely wigged out on someone there; I don't remember who. One of the many things I was trying to get fixed/resolved was a heat filter. The JJ's had a holder for a round infra-red filter, just behind the trap. I kept ordering it, saying, "They are round shaped." and kept getting square-shaped ones. This went on about 6-7 times within a year. I finally said something like, "Dope! Don't you know the difference between square and round!" There were several problems that went on and on like this. I had another exciting go-around with a repair/exchange intermittent that would'nt work, but that another story... I must say that I liked the design of Centurys, just not the quality of the manufacturing or some of the support people that were working there in 1982-87. I'm glad you guys bought them, because I think it's a good simple design if made correctly. I got that little wedge-shaped thing to align the trap to the intermittent sprocket- but it didn't help with the gate at all. If you feel like looking up my projectors, I think the ser no.s were 152 and 153, or 182 and 183- I can't remember which.
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